In Long Term Evolution (LTE), a physical control format indicator channel (PCFICH) is used as a control channel. The PCFICH notifies a receiving terminal of the number of orthogonal frequency division multiplexing (OFDM) symbols with which a physical downlink control channel (PDCCH) is to be transmitted. The receiving terminal is notified of the number of OFDM symbols as a control format indicator (CFI). A code word of the CFI consists of 3 bits. When a value of the CFI is 1, the code word is expressed as “011” in binary. When the value of the CFI is 2, the code word is expressed as “101” in binary. When the value of the CFI is 3, the code word is expressed as “110” in binary. When the value of the CFI is 4, the code word is expressed as “000” in binary.
On the transmission side, the CFI is further converted to repetition codes to generate a 32-bit transmission signal sequence. The receiving terminal receives the 32-bit transmission signal sequence, decodes the repetition codes of the 32-bit transmission signal sequence, and then decodes the CFI. As a decoding method, soft decision decoding is commonly used. In the case of the PCFICH, distances between decoding results of the repetition codes and respective code word candidates are calculated, and the CFI corresponding to the code word candidate having the highest correlation is produced as a decoding result.
Note that, the PDCCH is a control channel for transmitting control information for receiving a physical downlink shared channel (PDSCH) by the receiving terminal. The PDSCH is a channel for transmitting packet data to the receiving terminal.
FIG. 6 illustrates the CFI that has been converted to the repetition codes. First, a 32-bit sequence CPCFICHsoftbit (i) for decoding the repetition codes is divided into eleven units of 3 bits. Note, however, that the 11th unit contains 2 bits. Each of the bits in a unit corresponds to a code word of the CFI. In decoding the repetition codes, the eleven units are summed for each of the bits. It is assumed that the decoding results of the repetition codes are first to third sums A, B, and C, the first to third sums A, B, and C are expressed by Equations (1):A=ΣCPCFICHsoftbit(i) where i=0, 3, 6, 9, . . . 30B=ΣCPCFICHsoftbit(i) where i=1, 4, 7, 10, . . . 31C=ΣCPCFICHsoftbit(i) where i=2, 5, 8, 11, . . . 29  Eqs. (1)
In the soft decision decoding, distances from the code word candidates are taken. At this time, ‘0’ and ‘1’ each constituting 1 bit of the code word are converted to +1 and −1 for transmission, respectively, and hence Euclidean distances are taken on the receiving terminal side. When the Euclidean distances from the respective code word candidates are denoted as first to fourth Euclidean distances PCFI1, PCFI2, PCFI3, and PCFI4, respectively, the first to fourth Euclidean distances PCFI1, PCFI2, PCFI3, and PCFI4 are expressed by Equations (2):PCFI1=A−B−C PCFI2=−A+B−C PCFI3=−A−B+C PCFI4=A+B+C  Eqs. (2)
From the plurality of distances, the CFI corresponding to the code word candidate having the highest correlation is decoded by the computation expressed by Equation (3):
                    CFI        =                  {                                                    1                                                                                  if                    ⁢                                                                                  ⁢                                          P                                              CF                        ⁢                                                                                                  ⁢                        11                                                                              =                                      Max                    ⁡                                          (                                                                        P                                                      CF                            ⁢                                                                                                                  ⁢                            11                                                                          ,                                                  P                                                      CF                            ⁢                                                                                                                  ⁢                            12                                                                          ,                                                  P                                                      CF                            ⁢                                                                                                                  ⁢                            13                                                                          ,                                                  P                                                      CF                            ⁢                                                                                                                  ⁢                            14                                                                                              )                                                                                                                          2                                                                                  if                    ⁢                                                                                  ⁢                                          P                                              CF                        ⁢                                                                                                  ⁢                        12                                                                              =                                      Max                    ⁡                                          (                                                                        P                                                      CF                            ⁢                                                                                                                  ⁢                            11                                                                          ,                                                  P                                                      CF                            ⁢                                                                                                                  ⁢                            12                                                                          ,                                                  P                                                      CF                            ⁢                                                                                                                  ⁢                            13                                                                          ,                                                  P                                                      CF                            ⁢                                                                                                                  ⁢                            14                                                                                              )                                                                                                                          3                                                                                  if                    ⁢                                                                                  ⁢                                          P                                              CF                        ⁢                                                                                                  ⁢                        13                                                                              =                                      Max                    ⁡                                          (                                                                        P                                                      CF                            ⁢                                                                                                                  ⁢                            11                                                                          ,                                                  P                                                      CF                            ⁢                                                                                                                  ⁢                            12                                                                          ,                                                  P                                                      CF                            ⁢                                                                                                                  ⁢                            13                                                                          ,                                                  P                                                      CF                            ⁢                                                                                                                  ⁢                            14                                                                                              )                                                                                                                          4                                                                                  if                    ⁢                                                                                  ⁢                                          P                                              CF                        ⁢                                                                                                  ⁢                        14                                                                              =                                      Max                    ⁡                                          (                                                                        P                                                      CF                            ⁢                                                                                                                  ⁢                            11                                                                          ,                                                  P                                                      CF                            ⁢                                                                                                                  ⁢                            12                                                                          ,                                                  P                                                      CF                            ⁢                                                                                                                  ⁢                            13                                                                          ,                                                  P                                                      CF                            ⁢                                                                                                                  ⁢                            14                                                                                              )                                                                                                                              Eq        .                                  ⁢                  (          3          )                    
Japanese Unexamined Patent Application Publication (JP-A) No. 2006-173724 (hereinafter, referred to as “Patent Document 1”) discloses a decoding apparatus capable of significantly reducing an amount of calculation in the trellis or turbo-trellis coded modulation scheme. The trellis decoding apparatus disclosed in Patent Document 1 includes a decoder, a hard decision block, and a high-order bit decoder. The decoder comprises squared Euclidean distance calculation means, branch metric calculation means, and forward path metric calculation means. The squared Euclidean distance calculation means specifies a representative metric with respect to a received signal by referring to a lookup table for specifying a candidate signal point to calculate a squared Euclidean distance. The branch metric calculation means calculates a branch metric obtained by inverting the sign of the squared Euclidean distance. The forward path metric calculation means calculates a path metric in time series. The hard decision block performs a hard decision on the path metric to determine 3 low-order bits. The high-order bit decoder decodes high-order bits with the 3 low-order bits and the received signal.
However, in the LTE, the CFI having the value of 4 is never transmitted under the specification. In the case of a Multimedia Broadcast and Multicast Service (MBMS) single frequency network (MBSFN) sub-frame, the CFI having the value of 3 is never transmitted, either. In such case, simple soft decision decoding of the CFIs as in Equation (3) may lead to deterioration of the error rate and an increase in circuit scale.
Specifically, in the case where there is data that is never transmitted and the data may be discriminated in the receiving terminal, when the received transmission signal sequence is simply decoded, the data that is never transmitted may be produced as the decoding result to deteriorate the error rate.